Multiple access system and method for multibeam digital radio systems

ABSTRACT

A multiple-access digital radio communication system and method with communication links between user terminal transmitters and central node with a receiver system including a multibeam antenna. User terminal transmitters assigned to one beam coverage region use mltiple access channels that are mutually orthogonal for transmitting digital message information. These multiple access channels are reused in adjacent and other beam coverage regions. Error-correction coding ( 20 ), interleaving ( 21 ), and a single-axis modulator ( 24 ) are used in the user transmitter to increase resistance to potential interference from user terminal transmitters in other coverage regions. At the receiver, an adaptive processor ( 28 ) such as an equalizer or sequence estimator is used to combine multiple antenna beam signals ( 27 ) to produce a combined signal associated with each user. Deinterleaving ( 30 ) and error-correction decoding ( 31 ) of the combined signal is used to complete the recovery of the digital message information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International application No.PCT/US00/12802 filed May 11, 2000, which in turn claims the benefit ofboth U.S. provisional application Ser. No. 60/137,028 filed Jun. 1, 1999and U.S. provisional application Ser. No. 60/141,198 filed Jun. 25,1999.

FIELD OF THE INVENTION

This invention relates generally to multiple access communication in adigital radio system, and more particularly to improvements in themultiple access communication of fixed remote user terminals and/ormobile user terminals with a central node having a multibeam antenna andassociated receiver.

BACKGROUND OF THE INVENTION

Multiple access radio systems provide communication services for fixedremote user terminals and/or mobile user terminals. Examples of multipleaccess radio systems include land mobile radio networks, cellular mobileradio networks, and wideband radio networks between fixed subscribersand one or more central nodes, which may use a multibeam antenna forincreasing system capacity and improving communications quality. Thereverse link or uplink in a multiple access radio system is acommunications link between a fixed remote or mobile user terminal and acentral node, which can be located at either a fixed location on theEarth in a terrestrial radio system or as part of an orbiting satellitein a satellite radio system.

Digital radio systems transmit and receive digital message information,e.g., computer or Internet data. Alternatively, digital radio systemsaccept analog message information, e.g., voice or video data, andconvert this analog information to a digital format during transmissionand reception. Accordingly, a fixed remote or mobile user terminaltransmits message information in a digital format using an uplink to acentral node, where a multibeam antenna and associated receiver processreceived signals to extract user message information. In some satelliteradio systems, the receiver processing is divided between a satelliterepeater and a ground-based station processor.

User terminals within the same beam coverage region generally avoidmutual interference through the use of some form of multiple accessscheme. Conventional multiple access radio services use FrequencyDivision Multiple Access (FDMA), Time Division Multiple Access (TDMA),Code Division Multiple Access (CDMA), or some combination thereof.Generally, FDMA separates users into different frequency subbands; TDMAseparates users into different time intervals or slots; and, CDMAseparates users by assigning different signature waveforms or spreadingcodes to each user. These CDMA spreading codes can be either orthogonal,i.e., there is no interference between synchronized users, orquasi-orthogonal, i.e., there is some small interference between users.FDMA and TDMA are orthogonal multiple access (OMA) schemes because withideal frequency filters and synchronization there is no mutualinterference. Another example of an OMA system is CDMA with orthogonalspreading codes. Quasi-Orthogonal Multiple Access (QOMA) systems includeCDMA with quasi-orthogonal spreading codes and FDMA/TDMA with randomizedfrequency hopping.

In both orthogonal and quasi-orthogonal multiple access systems, themultiple access channels are usually assigned by a centralizedcontroller which may make assignments for a single central node beamcoverage region or the assignments may cover the beam coverage regionsof multiple central nodes. The assignments to the user terminals arenormally transmitted in time division with downlink message information.After synchronization, user terminals can extract the channel assignmentdata from the downlink messages.

For an isolated beam, an OMA scheme generally provides a larger systemcapacity than a QOMA scheme. However, when other beams are taken intoaccount, practical systems often use QOMA schemes for reducinginterference between users to acceptable levels.

Interference between a user on one beam and users on other beams isnormally reduced by user/beam cross-channel attenuation. However, in OMAradio systems, such cross-channel attenuation usually does not reduceinterference enough to allow the reuse of the same orthogonal waveformor channel in adjacent beams. Instead, channel management is typicallyrequired for determining when a multiple access channel can be reused inanother beam depending on an allowable threshold of the user/beamcross-channel attenuation. This leads to a reuse factor that is lessthan 1. The reuse factor of a multiple access channel is defined as thenumber of user terminal assignments in different beam coverage regionsdivided by the total number of beam coverage regions. Because thecapacity of a multiple access system is proportional to the averagevalue of the reuse factor with respect to all the multiple accesschannels, it is desirable to make the reuse factor for each multipleaccess channel as large as possible subject to interference constraints.

Practical limitations on multibeam antennas typically cause the reusefactor in cellular OMA systems to vary between ⅓ and 1/12.

In contrast, in a QOMA radio system, e.g., the uplink of a CDMA radiosystem in the IS-95 standard, the reuse factor can be unity because thecombination of user/beam cross-channel attenuation and spreading codeinterference protection is sufficient to keep mutual interferencebetween users in different beams to adequately small levels. However,one drawback is that a QOMA radio system generally has a theoreticalcapacity that is less than that of an OMA radio system.

Conventional multiple access digital radio systems provide means forerror-correction coding/decoding of message information, means forinterleaving/deinterleaving the message information, and a transmissionformat for the message information that includes reference signalsub-blocks. The reference signal is generated at both the user terminaland the central node and used by the central node receiver for obtainingchannel parameters to aid in demodulating a user signal. The insertionof a known reference signal in time multiplex with the transmittedmessage information is described in “An Adaptive Receiver for DigitalSignaling through Channels with Intersymbol Interference”, J. G. Proakisand J. H. Miller. IEEE Transactions on Information Theory, vol. IT-15,No. 4, July 1969 and in U.S. Pat. No. 4,365,338. Error-correction codingadds redundancy to message information in a prescribed manner so thattransmission errors may be corrected with a decoder at the receiver. Thepurpose of the interleaver/deinterleaver is to randomize thesetransmission errors at the decoder input so as to make the decoder morecapable of correcting them.

Further, the message information conventionally undergoes quadraturetransmission, wherein two carriers in phase quadrature to one another,e.g., cos ω_(c)t and sin ω_(c)t, are simultaneously transmitted usingthe same channel. Quadrature transmission is an example of a multisymbolsignaling scheme, wherein pluralities of successive binary digits ofuser data are combined to form symbols to be transmitted. Suchmultisymbol signaling schemes are typically used to reduce the bandwidthrequired to transmit the user data. Quadrature amplitude modulation(QAM) is an example of a general multisymbol signaling scheme, whereinmultilevel amplitude modulation is applied separately on each of the twoquadrature carriers.

Some conventional digital radio systems use adaptive equalizers forcombining multipath signals and reducing intersymbol interference.Adaptive equalizers have also been proposed for use with a multibeamreceiver for reducing interference from other users.

MMSE Equalization of Interference on Fading Diversity Channels, PeterMonsen, IEEE Conference on Communications, Conference Record, Vol. 1,Denver, Colo., June 1981, pp. 12.2-1–12.2-5, describes an adaptiveequalizer that combines multipath signals and reduces intersymbol andother user interference. The total interference is included in an errorsignal whose mean square value is minimized. Transmission of a timedivision multiplexed reference that is known at the receiver is alsodescribed.

U.S. Pat. Nos. 4,112,370 and 4,644,562 disclose the cancellation ofinterference in multibeam antennas as a generalization of thecancellation of interference in dual-polarized antennas.

U.S. Pat. No. 5,680,419 discloses adaptive sequence estimationtechniques that can be used with a multibeam antenna for cancelinginterference. Adaptive Equalization and Interference Cancellation forWireless Communication Systems, B. C. W. Lo and K. B. Letaief, IEEETrans. Comm., vol. 47, no. 4, April 1999, pp. 538–545 discloses in amultiantenna application a maximum likelihood sequence estimationtechnique that uses a reference signal of the desired user in order todetect a user signal in the presence of intersymbol interference andother user interference. Although either an equalizer or a sequenceestimator or a combination of both can be used for adaptive processing,the equalizer is generally preferred because it is not as complex as thesequence estimator.

Other relevant patent documents and publications include U.S. Pat. No.5,838,742; Dynamic Channel Assignment in High-Capacity MobileCommunications Systems, D. C. Cox and D. O. Reudick, Bell System. Tech.Journal, vol. 51, pp. 1833–2857, July–August 1971; MMSE Equalization onFading Diversity Channels, P. Monsen, IEEE Transactions onCommunications, vol. COM-32, No. 1, pp. 5–12, January 1984; LinearMultiuser Detectors for Synchronous Code-Division Multiple AccessChannels, R. Lupas and S. Verdu, IEEE Transactions on InformationTheory, vol. IT-35, No. 1, pp. 123–136, January 1989; DecorrelatingDecision-Feedback Multiuser Detector for Synchronous Code-DivisionMultiple Access Channels, A. Duel-Hallen, IEEE Transactions onCommunications, vol. COM-41, No. 2, pp. 285–290, February 1993; A Familyof Multiuser Decision Feedback Detectors for Asynchronous Code-DivisionMultiple Access Channels, A. Duel-Hallen, IEEE Transactions onCommunications, vol. COM-43, Nos. 2,3,4, February–April 1995;Information-Theoretic Considerations for Symmetric, Cellular, MultipleAccess Fading Channels-Part I, S. Shamai and A. D. Wyner, IEEETransactions on Information Theory, vol. 43, No. 6, pp 1877–1894,November 1997; and, Mobile Station-Base Station Compatibility Standardfor Dual-Mode Wideband Spread Spectrum Cellular System, EIA/TIA IS-95,1992.

Although the techniques described above have been used for improvingcommunications quality and increasing the capacity of multiple accesscommunication systems, it has been recognized that the capacity of OMAsystems is limited because its multiple access channels have reusefactors less than 1. It has also been recognized that the capacity ofQOMA systems is limited because its theoretical capacity is less thanthat of a corresponding OMA system.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of the inventionto provide a multiple access communication system and method withincreased channel capacity and improved communications quality.

Another object of the invention is to provide a multiple accesscommunication system and method that is orthogonal in each beam coverageregion and has a channel capacity greater than that of conventionalquasi-orthogonal multiple access communication systems.

Still another object of the invention is to provide an orthogonalmultiple access communication system and method that has a reuse factorof unity.

The foregoing and other objects are achieved in a multiple accesscommunication system including a plurality of user terminals, eachincluding a user terminal transmitter; and a central node including amultibeam antenna and associated receiver for receiving digital messageinformation transmitted by the user terminal transmitters. The receiverincludes a multibeam antenna that receives multiple access signals fromthe user terminal transmitters. The multibeam antenna produces beamcoverage regions in which user terminals are located. User terminaltransmitters are associated with a beam coverage region, and those userterminal transmitters associated with a beam coverage region employmutually orthogonal multiple access waveforms. User transmitterterminals associated with other beam coverage regions can reuse the sameorthogonal waveforms from the mutually orthogonal waveform set.

In a preferred embodiment, the user terminal transmitter includes acoding unit for providing error-correction coding of the digital messageinformation, an interleaving unit for distributing the error-correctioncoded message information, a multiplexer for multiplexing user referencesignals with the error-correction coded message information, and amodulator for modulating the multiplexed signal to a single radiofrequency axis for subsequent transmission as a respective multipleaccess signal, wherein each of the respective multiple access signalsassociated with a beam coverage region employ an orthogonal waveformfrom a mutually orthogonal waveform set. The modulator may also apply arandom phase shift to a group or packet of multiplexed data containingat least one reference signal subblock. In some systems the random phaseshift can be omitted because user terminal motion provides a similareffect.

In this preferred embodiment at the central node, a receiver includes amultibeam antenna for receiving respective multiple access signals fromthe user terminal transmitters, an adaptive processor for each user thatprocesses the received beam signals and the reference signals to combinethe received beam signals and reduce other user interference, and adeinterleaver and decoder for each user to recover the digital messageinformation from the combined signal.

Random phase modulation and channel variations randomize the receivedpacket phase, which improves the deinterleaving/decoding operation. Theerror-correction coded information is interleaved before transmission sothat after deinterleaving at the receiver, the phase modulation ofsuccessive error-correction coded symbols is different. The interleavingthus improves the error-correcting capability of the decoder andincreases the interference protection. In general every user may employa unique reference signal so that the receiver can extract channelinformation for that user by generating a replica of the uniquereference signal and processing it with the received antenna beamsignals. Since users within the same beam coverage region are assignedchannels from a mutually orthogonal set, these users may employ the samereference signal. At the receiver there is an adaptive processor foreach user and this adaptive subsystem processes the antenna beam signalsthat contain significant adjacent beam interference. In the preferredembodiment the adaptive processor is a multibeam equalizer thatminimizes a mean square error by means of an estimated gradientalgorithm. The error contains all the channel effects including additivenoise, other user interference, and multipath reflection effects. Inthis preferred embodiment only the user reference signal for the desireduser is required in the estimated gradient algorithm. In an alternativeembodiment the adaptive processor solves a set of simultaneous equationsfor each received data group. The solution of these equations providesthe processor settings for that received group. The user referencesignal for a respective beam coverage region and user reference signalscorresponding to interfering beams are employed to determine theadaptive processor parameter settings in this alternative embodiment.

Still further objects and advantages will become apparent from aconsideration of the ensuing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the followingmore detailed description and accompanying drawings in which:

FIG. 1 is a diagram of a packet for transmission in the communicationsystem of the present invention;

FIG. 2 is a diagram of beam coverage regions containing user terminaltransmitters that transmit information to a receiver in the presentinvention.

FIG. 3 is a functional block diagram of a central node connected to anassignment controller in the present invention.

FIG. 4 is a functional block diagram of a user terminal transmitter inthe communication system of the present invention; and

FIG. 5 is a functional block diagram of a receiver at a central node inthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the transmission format for a packet 10 of user messageinformation to be sent over a reverse link or an uplink in a multipleaccess radio system between a fixed remote user terminal or a mobileuser terminal and a central node, which may be located at either a fixedlocation on the Earth in a terrestrial radio system or as part of anorbiting satellite in a satellite radio system.

The packet 10 includes a user reference signal that is a block ofreference data 11, which is inserted into the packet 10 at the userterminal. In particular, the reference data 11 includes a sequence ofknown data symbols (not shown) that may be inserted into the packet 10either as a contiguous block as depicted in FIG. 1 or in somedistributed manner. The sequence of data symbols and the manner in whichthey are inserted into the packet 10 are known at the central node foreach user in the multiple access radio system whose message informationis to be processed at that node. Further, the reference data 11 is usedfor determining adaptive processor parameter settings at the centralnode, as described in further detail below.

The packet 10 also includes interleaved and coded data 12, which isrepresentative of processed user message information in digital form.Finally, the packet 10 may include other system or user information (notshown) in addition to the reference data 11 and the interleaved andcoded data 12.

As shown in FIG. 2 the multiple access radio system according to thepresent invention includes user terminal transmitters 13 that areincluded in user terminals and that have associated digital radiocommunication links, i.e., reverse links or uplinks, to a receiver 14 atthe central node. The receiver 14 includes a multibeam antenna thatproduces beam coverage regions 15 that are numbered 1–7 in this example.Further, user terminal transmitters 13 are associated with beam coverageregions 15. The beam coverage regions typically overlap because antennabeams do not have ideal cutoff transitions. A user terminal transmitter13 is usually assigned to a user terminal beam coverage region 15depending on geographic location and antenna beam responsecharacteristics. Typical user/beam cross-channel interference betweenuplink transmissions as a result of overlap of the antenna beam patternsoccurs in adjacent beam coverage regions. This adjacent beaminterference is illustrated in FIG. 2 by dark arrows between the beamcoverage region pairs 1–2, 1–3, 1–7, 2–3, 2–4, 2–5, 2–6, 2–7, 3–4, 4–5,5–6, and 6–7.

User terminal transmitters 13 associated with the same beam coverageregion 15 are assigned orthogonal multiple access (OMA) channels from amutually orthogonal set. Examples of OMA schemes that may be used withthe multiple access radio system of the present invention includeFrequency-Division Multiple Access (FDMA), Time-Division Multiple Access(TDMA), and Orthogonal-Waveform Code-Division Multiple Access (OCDMA),and various combinations thereof. In the present invention the samemultiple access channel may be reassigned in all other beam coverageregions for a reuse factor of unity. The assignment of channels to theuser terminal transmitter 13 can be accomplished by transmittingassignment data to the user terminal from either the central node or acentralized location that includes multiple central nodes. Assignmentdata may be time division multiplexed with downlink message transfers ortransmitted on a separate channel to the user terminal. With referenceto FIG. 3, in the preferred embodiment the multiple access channelassignments are produced by an assignment controller 16 that is eithercollocated with the central node 17 or is connected to the central node17 with a communication link (indicated as a double-arrow line in FIG.3) to transfer assignment data to an assignment processor 18. Theassignment processor 18 formats assignment data for downlink transfer touser terminals via a central node transmitter 19 and it receives statusinformation from the receiver 14 to be described subsequently thatrecovers uplink message information. This status information is passedon to the assignment controller 16 by the assignment processor 18.

Each user terminal transmitter 13 associated with a particular beamcoverage region is assigned a multiple access channels from a mutuallyorthogonal set. Thus user terminal transmitters 13A and 13B associatedwith the same beam coverage region do not interfere with each otherunder ideal transmission conditions. The user terminals 13 and 13A (or13 and 13B) shown in beam coverage regions numbered 1 and 7,respectively, may be assigned the same multiple access channel by theassignment controller 16 and thus would interfere with each other underideal transmission conditions. For nonideal transmission conditions suchas multipath reflections there can be both intrabeam and interbeaminterference which must be compensated for by an adaptive processor inthe receiver 14. Because of adjacent beam interference the interbeaminterference is typically more severe in multipath environments of anOMA system because there is no orthogonal channel protection.

Before describing in detail a user terminal transmitter 13 and areceiver 14 in accordance with the present invention, it should beunderstood that the present invention includes conventionalcommunication system components, e.g., error-correction coder/decoder,interleaver/deinterleaver, multiplexer/demultiplexer,modulator/demodulator, and adaptive processor, which perform tasksrelated to the transmission and/or reception of user messageinformation. Because these communication system components areconventional and known to those skilled in this art, the user terminaltransmitter 13 and the receiver 14 have been described through the useof functional block diagrams, wherein each block is representative ofone of these conventional communication system components. For theadaptive processor, a preferred and alternative construction isidentified below.

FIG. 4 shows a preferred embodiment of a user terminal transmitter 13according to the present invention. User message information to betransmitted on an uplink to a receiver 14 may initially be in eitheranalog or digital form. However, the user message information ispreferably converted, if necessary, into digital form before beingprovided as a digital input to a coder 20, which adds redundancy in theform of an error-correction code, thereby causing the digitaltransmission rate of coded data at the output of the coder 20 to begreater than the digital input rate at the input of the coder 20. Itshould be noted that the type, the subclass, and the parameters relatedto the error-correction code are not critical to the present invention.In a binary communication system an example of an error-correctioncoding technique would be the rate ½, constraint length 7, convolutionalcode with generators 133,171.

The coder 20 provides the coded data to an interleaver 21, whichdistributes the coded data among multiple packets in a predeterminedmanner. In a preferred embodiment, the coded data is distributed amongthe multiple packets as follows. If there are N symbols per packet, thenthe N symbols are evenly distributed over N packets; e.g., symbol 1 goesin packet 1, symbol 2 goes in packet 2, and so on, until symbol N goesin packet N, and then the process is repeated until all N packets arefull. However, it should be understood that the interleaver 21 maydistribute the coded data into the multiple packets in other ways andstill achieve a reuse factor of 1 in the uplink of the OMA system.

A reference generator 22A locally produces the sequence of known datasymbols included in the reference data 11 mentioned above, and thenprovides the reference data 11 to a packet multiplexer 23. The block ofreference data 11 that is inserted into packet 10 can be unique to eachuser or it may be the same for each user in a beam coverage region 15but unique relative to users in other beam coverage regions 15 in themultiple access radio system. Similarly, the interleaver 21 provides theinterleaved and coded data 12 to the packet multiplexer 23, which thengenerates packets having the general form shown in FIG. 1. As alsomentioned above, each packet may include other system or userinformation in addition to the reference data 11 provided by thereference generator 22A and the interleaved and coded data 12 providedby the interleaver 21.

Next, the packet multiplexer 23 sequentially provides the generatedpackets to a single-axis modulator 24, which converts the packetizeddata to a multiple access signal suitable for transmission over anuplink to a receiver 14 using a radio frequency (RF) channel. Inparticular, the packets generated by the packet multiplexer 23 areconverted by the single-axis modulator 24 to use only one of twoquadrature carriers, e.g., cos ω_(c)t or sin ω_(c)t. For example, thesingle-axis modulator 24 may use, e.g., pulse amplitude modulation (PAM)for applying multilevel amplitude modulation of the user data on one ofthe two quadrature carriers.

Although the use of PAM in the single-axis modulator 24 reduces thenumber of possible bits per transmitted symbol by a factor of two whencompared with a modulator using quadrature transmission, e.g., QAM, thetheoretical system capacity in this multibeam application is expected toincrease because the single-axis modulator 24 is expected to make theuser terminal transmitted signal more resistant to interference whenadaptive interference reduction is accomplished at the receiver 14. Thisincreased resistance to interference is also largely responsible for thereuse of multiple access channels in adjacent beam coverage regions.

Further, the single axis modulator 24 preferably applies a pseudo-randomphase shift to each generated packet. This applied phase shift is fixed,i.e., constant, for the duration of the packet 10. In multiple accessradio systems with mobile user terminals the application of a variablephase modulation may be unnecessary because user motion may produce thedesired phase shift. Finally, the single axis modulator 24 provides themultiple access signal to an antenna 25 for transmission over an uplinkto a receiver 14 using an RF channel.

FIG. 5 shows a preferred embodiment of a receiver 14 according to thepresent invention. The receiver 14 includes a multibeam antenna 26,which receives the multiple access signals produced by the single axismodulator 24 in each user terminal transmitter and transmitted over theRF channel. The multibeam antenna 26 receives multiple access signals ona plurality of beams, each beam including a set of users assigned torespective OMA channels. The same orthogonal channel may be reused indifferent beams because of the increased interference protectionprovided by the combination of error-correction coding/decoding,interleaving/deinterleaving, single-axis demodulation and adaptiveprocessor combining so that a reuse factor of unity is achieved. Theoutput of the multibeam antenna 26 is a set of multibeam antennasignals.

If the multibeam antenna has B beams and B antenna beam signals, thenfor a particular user there are M antenna beam signals that containusers with significant interference, wherein M≦B. The choice of M isdependent upon the required communications quality and the requiredcomplexity of the receiver 14. Accordingly, the multibeam antenna 26provides the B antenna beam signals to B RF demodulators 27.

For the particular user mentioned above, a subset of the RF demodulators27 provide M demodulated signals to an M-dimensional adaptive processor28 for beam combining and other user interference reduction. In apreferred embodiment, the subset of the RF demodulators 27 converts theantenna beam signals provided by the multibeam antenna 26 from the RFchannel to digital form at baseband for subsequent processing by theadaptive processor 28.

The adaptive processor 28 processes multiple demodulated signals andpossibly previously detected digital symbols from the particular userand the other users, thereby generating a combined signal with reducedinterference from the other users. The adaptive processor 28 may takethe form of an adaptive equalizer that minimizes some error criterion,or an adaptive sequence estimator that finds the most likely transmitteddigital symbol sequence for the particular user, or some combination ofboth. Examples of adaptive equalizers that might be used in thismultibeam application include linear minimum mean square error (MMSE)receivers, decorrelation detectors, and decision-feedback detectors. Thepreferred embodiment of an adaptive equalizer is described in “MMSEEqualization of Interference on Fading Diversity Channels”, P. Monsen,IEEE Transactions on Communications, vol. 32, no. 1, January 1984[hereafter denoted by MMSE Equalization], the disclosure of which ishereby incorporated by reference. An example of adaptive sequenceestimation in this multibeam application is described in AdaptiveEqualization and Interference Cancellation for Wireless CommunicationSystems, B. C. W. Lo and K. B. Letaief, IEEE Trans. Comm., vol. 47, no.4, April 1999, pp. 538–545, the disclosure of which is herebyincorporated by reference.

In the preferred embodiment the adaptive processor 28 is adapted byusing a replica of the reference data for a particular user. To thisend, the reference generator 22B provides a user-identifying replica ofthe reference signal for a particular user, e.g., reference data 11, tothe adaptive processor 28. The adaptive processor 28 exploits theuser-identifying replica to adapt its parameters and then generates acombined signal that corresponds to a particular user from the M antennabeam signals.

The combining in the preferred embodiment is accomplished by using theMMSE equalizer described in MMSE Equalization, above. The adaptation ofthis equalizer to minimize a mean square error containing noise andinterference is accomplished with the user-identifying replica that isknown to the receiver 14. As described in MMSE Equalization, thisadaptation takes place when the reference signal is present in thereceived signal corresponding to a desired user. The error signal isderived from the difference between the combined signal and theuser-identifying replica. The adaptation of the MMSE equalizer describedin MMSE Equalization is accomplished in the preferred embodiment withthe Least-Mean-Squares (LMS) algorithm. The LMS algorithm is a wellknown estimated-gradient algorithm that has been applied in manyequalizer applications. This algorithm is applied to the adaptation of alinear equalizer in “An Adaptive Receiver for Digital Signaling throughChannels with Intersymbol Interference” J. G. Proakis and J. H. Miller,IEEE Trans. on Information Theory, vol. IT-15, No. 4, July 1969 and isapplied to the adaptation of a decision-feedback equalizer in “FeedbackEqualization for Fading Dispersive Channels”, P. Monsen, IEEE Trans. OnInformation Theory, vol. IT-17, pp. 55–64, January 1971. In this LMSalgorithm with application to the MMSE equalizer of MMSE Equalization,the error signal, referenced above, is multiplied by each of the Mantenna beam signals and each equalizer weight is updated by a fractionof the multiplication product corresponding to its antenna beam signal.The LMS algorithm is an estimated gradient algorithm in that it searchesfor the optimum mean-square error solution by making steps in amultidimension space that correspond to the negative of the estimatedgradient. The combined signal is then produced by multiplying theequalizer weights and antenna beam signals and adding up the M products.Interference and noise are both reduced because the mean square value ofthe error signal has been minimized. This equalizer can be extended toinclude multipath dispersion and cancellation of previous user decisionsas described in MMSE Equalization.

In an alternative embodiment, the combining is accomplished by using notonly the replica of the reference signal of the desired user butreplicas of reference signals for interfering users as well. For thedesired user and each potential user interferer in M-1 other beamscoverage regions, the adaptive processor 28 performs M² correlations ofthe user-identifying replicas from a plurality of reference generators22B and antenna beam signals from RF demodulators 23 to obtain user/beamcross-channel values A_(IJ) for user J transmitting to beam I. Thevalues A_(IJ) are in general complex. As an example of possibleuser/beam cross-channel interference consider FIG. 2 which shows a groupof beam coverage regions 15 which contain user terminal transmitters 13that send message information to the receiver 14. The receiver 14, asdescribed above with reference to FIG. 5, contains, for example, a sevenbeam antenna that produces the beam coverage regions 15 or cellsnumbered 1–7. Interference between user terminal signals is mostpronounced between users in adjacent cells. Since the users in each cellare assigned multiple access channels from a mutually orthogonal set,the first order interference in the absence of multipath distortion isonly from users with the same multiple access channel but located in anadjacent cell. Analysis of this type of configuration has shown that theworst interference situation occurs for a user in a ring cell, i.e.,1,3–7, rather than the center cell 2. If, for example, the receiver 14is trying to recover the message information for a user in cell 1 thenthe minimum user/beam cross channel values of interest (cells 2, 3, and7 being adjacent to cell 1) are A_(11,) A_(12,) A_(21,) A_(13,) A_(31,)A_(17,) A_(71,) which corresponds to M=4 in a B=7 beam system. Betterinterference cancellation is obtained by increasing the value M≦B. ForM=7 in this example the user/beam cross channel values A_(IJ) for I,J=1,. . . 7 must be computed. The additional cross channel values in the M=7calculation increases the opportunity to cancel interference affectingthe user in cell 1. For a user in the center cell (cell 2, as shown inFIG. 2), the minimum nonzero user/beam cross channel values are A_(2J)and A_(J2), J=1,3–7, i.e. M=6. There is more interference in the centercell user example but theoretical capacity calculations show that thisinterference is exploited in the invention to realize improved capacityrelative to the ring cell example.

A solution for the optimum combining weights can be expressed in termsof the complex user/beam cross channel matrix A={A_(IJ), I, J=1,2, . . .M}, M≦7 in this example (it is understood that the indices may have beenrenumbered for convenience), and an estimate of the additive channelnoise power σ² in one quadrature axis at the multibeam antenna input.The equalizer weights are used in the combiner to form the real innerproduct for the L^(th) user

$\begin{matrix}{{s_{L} = {{\underset{\_}{w_{L}^{\prime}}x} = {\sum\limits_{l = 1}^{M}\;{w_{LI}y_{I}}}}},{L = 1},{\ldots\mspace{14mu} M}} & (1)\end{matrix}$

-   -   where W_(LI) is the I^(th) real equalizer weight for the L^(th)        user, y_(I) is the I^(th) matched antenna beam signal, and s_(L)        is the combined signal for the L^(th) user. The matched antenna        beam signals are real and are calculated in the matched filter        operation by

$\begin{matrix}{{y_{I} = {{Re}\left( {\sum\limits_{J = 1}^{M}\;{A_{JI}^{*}x_{J}}} \right)}},{I = 1},2,{\ldots\mspace{14mu} M}} & (2)\end{matrix}$

-   -   where Re (•) denotes the real part operation.

Direct calculation of the equalizer weight vector is accomplished bysolving the set of M simultaneous equations expressed in matrix/vectorformHw_(L)=e_(L)  (3)where

-   -   H=Re(A′A)+σ² I    -   e_(LI)=1 if L=I and 0 otherwise,    -   I=Identity Matrix,    -   σ²=channel noise power.

This direct calculation solution corresponds to the Minimum Mean SquareError (MMSE) solution determined in the preferred embodiment by means ofan LMS algorithm adaptation. The direct calculation solution isattractive when the number of interfering cells is not too large andmultipath distortion is minimal.

The matrix H is a symmetric matrix so that the simultaneous equations(3) can be solved by a Cholskey decomposition that is described innumerical computational texts such as “Least Square Estimation withApplication to Digital Signal Processing” by A. A. Giordano and F. M.Hsu, John Wiley and Sons, New York, N.Y., 1985, Chapter 3.3, thedisclosure of which is hereby incorporated by reference. In thisdecomposition method the matrix H is decomposed into G′G where G′ is thetranspose of G and G is a lower diagonal matrix, i.e. all the elementsin the matrix above the diagonal are zero.

In the special case where the additive channel noise power σ² is takenas zero, this MMSE solution reduces to a solution that cancels all theinterference, i.e. a zero-forcing solution.

It is understood that the use of additional equalizer weights in atapped-delay-line filter configuration could be used to compensate formultipath distortion or to cancel previously detected interference asdescribed in MMSE Equalization, referenced above. For this extendedweight example, the direct solution by solving a matrix/vector equationsuch as (1), above, is accomplished by extending the matrix and vectordefinitions to include the additional weights.

The combined signal is then provided to a packet demultiplexer 29, whichremoves the reference data, e.g., reference data 1, from the combinedsignal. The packet demultiplexer 29 provides the combined signal withoutthe reference data to a deinterleaver 30, which reverses theinterleaving of the coded data performed by the interleaver 21. Finally,the deinterleaver 30 provides the deinterleaved data to a decoder 31,which performs error-correction decoding, thereby producing a digitaloutput that is representative of the transmitted digital messageinformation. Subsequent processing, e.g., digital-to-analog conversion(not shown), of the digital output may be required to obtain analogmessage information, e.g., voice signals, of the particular user.

While embodiments have been shown and described in accordance with thepresent invention, it is understood that the same is not limited theretobut is susceptible to numerous changes and modifications as known to aperson skilled in this art. For example, while the invention has beendescribed as operating with a reuse factor having a value of unity, itwill, of course, be recognized by those skilled in the art that by notreusing a channel in one or even several, beam coverage regions, it ispossible to achieve at least some of the advantages of this invention(as compared to known systems where on average the multiple accesschannels have a reuse factor of ⅓ to 1/12^(th)). Such systems with reusefactors of less than one, but substantially improved over knowcommercial systems, e.g., have a reuse factor of at least ⅔, areintended to fall within the scope of the appended claims. Accordingly,the present invention should not be limited to the detail shown anddescribed herein but is intended to cover all such changes andmodifications as are obvious to one of ordinary skill in this art.

Therefore, the present invention should be limited only by the spiritand scope of the appended claims.

1. A method of communicating digital data information from a pluralityof user terminal transmitters located in a plurality of antenna beamcoverage regions to a receiver with a multibeam antenna, wherein eachuser terminal transmitter is associated with a beam coverage region,comprising the steps of: assigning a plurality of multiple accesschannels that belong to a mutually orthogonal set to the plurality ofuser terminal transmitters, each multiple access channel beingassociated with a reuse factor, which is defined by the number of userterminal transmitter assignments in different beam coverage regionsdivided by the total number of beam coverage regions, and at least onemultiple access channel is reused in all beam coverage regions so thatits reuse factor is unity; adding, at the user terminal transmitter,error-correction coding to the digital data information to provide codedinformation; interleaving, at the user terminal transmitter, the codedinformation among a plurality of data groups; inserting in time to eachdata group, at the user terminal transmitter, a reference signalassociated with the beam coverage region of the user terminaltransmitter to provide a multiplexed signal; modulating, at the userterminal transmitter, the multiplexed signal to a single radio-frequencyaxis with a multiple access waveform associated with the assignedmultiple access channel to provide a multiple access signal;transmitting, at the user terminal transmitter, the multiple accesssignal; receiving, at the multibeam antenna of the receiver, themultiple access signals from the plurality of user terminal transmittersso as to provide a plurality of antenna beam signals; combining, at thereceiver, the antenna beam signals to provide a combined signalassociated with a user terminal transmitter, thereby reducinginterference from user terminal transmitters assigned to different beamcoverage regions; deinterleaving and decoding, at the receiver, thecombined signal to recover the user terminal transmitter digital datainformation.
 2. The method of claim 1 wherein the modulating stepfurther includes: modifying in a random or pseudo-random manner thefixed phase shift of the single radio-frequency axis for the duration ofeach data group.
 3. The method of claim 2 wherein the combining stepfurther includes: generating a user-identifying replica of the referencesignal associated with a user terminal transmitter.
 4. The method ofclaim 3 wherein the combining step further includes: subtracting theuser-identifying replica from the combined signal to provide an errorsignal; minimizing the mean square value of the error signal.
 5. Themethod of claim 3 wherein the combining step further includes: producinga plurality of user-identifying replicas; correlating combinations ofuser-identifying replicas with antenna beam signals to provide aplurality of user/beam cross-channel values; calculating the real partof the product of cross-channel values and antenna beam signals toprovide a plurality of matched antenna beam signals; convertingcross-channel values into equalizer weights; summing the product ofequalizer weights and matched antenna beam signals, thereby producingthe combined signal.
 6. The method of claim 5 wherein the convertingstep further includes: solving a set of simultaneous equations with aCholskey decomposition.
 7. The method of claim 1 wherein the combiningstep further includes: generating a user-identifying replica of thereference signal associated with a user terminal transmitter.
 8. Themethod of claim 7 wherein the combining step further includes:subtracting the user-identifying replica from the combined signal toprovide an error signal; minimizing the mean square value of the errorsignal.
 9. The method of claim 7 wherein the combining step furtherincludes: producing a plurality of user-identifying replicas;correlating combinations of user-identifying replicas with antenna beamsignals to provide a plurality of user/beam cross-channel values;calculating the real part of the product of cross-channel values andantenna beam signals to provide a plurality of matched antenna beamsignals; converting cross-channel values into equalizer weights; summingthe product of equalizer weights and matched antenna beam signals,thereby producing the combined signal.
 10. The method of claim 9 whereinthe converting step further includes: solving a set of simultaneousequations with a Cholskey decomposition.
 11. A communication system forcommunicating digital data information from a plurality of user terminaltransmitters located in a plurality of antenna beam coverage regions toa multibeam antenna in a receiver at a central node, wherein each userterminal transmitter is associated with a beam coverage region,comprising: assignment controller, associated with the central node, forassigning a plurality of multiple access channels that belong to amutually orthogonal set to the plurality of user terminal transmitters,each multiple access channel being associated with a reuse factor, whichis defined by the number of user terminal transmitter assignments indifferent beam coverage regions divided by the total number of beamcoverage regions, and at least one multiple access channel is reused inall beam coverage regions so that its reuse factor is unity; and userterminal transmitter, disposed at the user terminal, for transmittingdigital data information in a multiple access signal to the receivercomprising: means for adding error-correction coding to the digital datainformation to provide coded information, and means for interleaving thecoded information among a plurality of data groups, and means forinserting in time to each data group a reference signal associated withthe beam coverage region of the user terminal transmitter to provide amultiplexed signal, and modulation means for modulating the multiplexedsignal to a single radio-frequency axis with a multiple access waveformassociated with the assigned multiple access channel to provide amultiple access signal; and receiver, disposed at the central node, forreceiving at the multibeam antenna the multiple access signals from theplurality of user terminal transmitters so as to provide a plurality ofantenna beam signals, comprising: summation means for combining theantenna beam signals to provide a combined signal associated with a userterminal transmitter, thereby reducing interference from user terminaltransmitters assigned to different beam coverage regions, and means fordeinterleaving and decoding the combined signal to recover the userterminal transmitter digital data information.
 12. A communicationsystem according to claim 11 wherein the modulation means furtherincludes: means for modifying in a random or pseudo-random manner thefixed phase shift of the single radio-frequency axis for the duration ofeach data group.
 13. A communication system according to claim 12wherein the summation means further includes: means for generating auser-identifying replica of the reference signal associated with a userterminal transmitter.
 14. A communication system according to claim 13wherein the summation means further includes: means for subtracting theuser-identifying replica from the combined signal to provide an errorsignal, and means for minimizing the mean square value of the errorsignal.
 15. A communication system according to claim 13 wherein thesummation means further includes: means for producing a plurality ofuser-identifying replicas, and means for correlating combinations ofuser-identifying replicas with antenna beam signals to provide aplurality of user/beam cross-channel values, and means for calculatingthe real part of the product of cross-channel values and antenna beamsignals to provide a plurality of matched antenna beam signals, andconversion means for converting cross-channel values into equalizerweights, and means for summing the product of equalizer weights andmatched antenna beam signals, thereby producing the combined signal. 16.A communication system according to claim 15 wherein the conversionmeans further includes: means for solving a set of simultaneousequations with a Cholskey decomposition.
 17. A communication systemaccording to claim 11 wherein the summation means further includes:means for generating a user-identifying replica of the reference signalassociated with a user terminal transmitter.
 18. A communication systemaccording to claim 17 wherein the summation means further includes:means for subtracting the user-identifying replica from the combinedsignal to provide an error signal, and means for minimizing the meansquare value of the error signal.
 19. A communication system accordingto claim 17 wherein the summation means further includes: means forproducing a plurality of user-identifying replicas, and means forcorrelating combinations of user-identifying replicas with antenna beamsignals to provide a plurality of user/beam cross-channel values, andmeans for calculating the real part of the product of cross-channelvalues and antenna beam signals to provide a plurality of matchedantenna beam signals, and conversion means for converting cross-channelvalues into equalizer weights, and means for summing the product ofequalizer weights and matched antenna beam signals, thereby producingthe combined signal.
 20. A communication system according to claim 19wherein the conversion means further includes: means for solving a setof simultaneous equations with a Cholskey decomposition.
 21. A methodfor receiving and processing digital information transmitted from aplurality of user terminals, located in plural beam coverage areas, at acentral node have a receiver and an associated multibeam antenna thatproduces the plural beam coverage areas, and each user terminaltransmits a user-identifying reference and digital data information thatis error-corrected coded and interleaved in a single-axis modulatedsignal associated with a multiple access channel that belongs to amutually orthogonal set to the receiver, comprising the steps of:assigning a multiple access channel to a first user terminal in a firstbeam coverage region and assigning the same multiple access channel to asecond user terminal in a second beam coverage region and the user/beamcross-channel attenuation between the first and second user terminals isunrestricted; receiving, at the multibeam antenna, the single-axismodulated signals from the first and second user terminals so as toprovide a plurality of antenna beam signals; generating auser-identifying replica of the reference signal associated with thefirst user terminal; processing the antenna beam signals and theuser-identifying replica to provide a combined signal associated withthe first user terminal, thereby reducing interference from the seconduser terminal; deinterleaving and decoding the combined signal torecover the first user terminal digital information.
 22. The method ofclaim 21 wherein the processing step further includes: subtracting theuser-identifying replica from the combined signal to provide an errorsignal; minimizing the mean square value of the error signal.
 23. Themethod of claim 21 wherein the generating step further includes:producing a second user-identifying replica associated with the seconduser terminal, and wherein the processing step further includes:correlating combinations of user-identifying replicas with antenna beamsignals to provide a plurality of user/beam cross-channel values;calculating the real part of the product of cross-channel values andantenna beam signals to provide a plurality of matched antenna beamsignals; converting cross-channel values into equalizer weights; summingthe product of equalizer weights and matched antenna beam signals,thereby producing the combined signal.
 24. The method of claim 23wherein the converting step further includes: solving a set ofsimultaneous equations with a Cholskey decomposition.
 25. A central nodewith a receiver including a multibeam antenna, which produces beamcoverage areas in which a plurality of user terminals are located andeach user terminal transmits a user-identifying reference and digitaldata information that is error-corrected coded and interleaved in asingle-axis modulated signal associated with a multiple access channelthat belongs to a mutually orthogonal set to the receiver, comprising:means for assigning a multiple access channel to a first user terminalin a first beam coverage region and assigning the same multiple accesschannel to a second user terminal in a second beam coverage region andthe user/beam cross-channel attenuation between the first and seconduser terminals is unrestricted, and means for receiving, at themultibeam antenna, the single-axis modulated signals from the first andsecond user terminals so as to provide a plurality of antenna beamsignals, and generating means for generating a user-identifying replicaof the reference signal associated with the first user terminal, andprocessing means for processing the antenna beam signals and theuser-identifying replica to provide a combined signal associated withthe first user terminal, thereby reducing interference from the seconduser terminal, and means for deinterleaving and decoding the combinedsignal to recover the first user terminal digital information.
 26. Themethod of claim 25 wherein the processing means further includes: meansfor subtracting the user-identifying replica from the combined signal toprovide an error signal, and means for minimizing the mean square valueof the error signal.
 27. The method of claim 25 wherein the generatingmeans further includes: means for producing a second user-identifyingreplica associated with the second user terminal, and wherein theprocessing means further includes: means for correlating combinations ofuser-identifying replicas with antenna beam signals to provide aplurality of user/beam cross-channel values, and means for calculatingthe real part of the product of cross-channel values and antenna beamsignals to provide a plurality of matched antenna beam signals, andconverting means for converting cross-channel values into equalizerweights, and means for summing the product of equalizer weights andmatched antenna beam signals, thereby producing the combined signal. 28.The method of claim 27 wherein the converting means further includes:means for solving a set of simultaneous equations with a Cholskeydecomposition.